of Edinburgh, Session 1885-86. 
645 
Or we may, as Tidy * recommends in a process for the estimation of 
organic matter in potable waters, add iodide of potassium, and 
estimate the liberated iodine with hyposulphite (thiosulphate) of 
sodium, adding starch solution to indicate the exact point at which 
to stop. Muter f uses a very similar method for the estimation of 
the nitrite in spirit of nitrous ether. I have carefully compared all 
these methods, and unhesitatingly give preference to that in which 
ammonio-ferrous sulphate is the reagent used, as it, while quite as 
accurate, is more easily employed than either of the other two 
Assuming a temperature of 60° E. to be taken as granted, the pro- 
cess, as I use it, requires the following solutions, the most conve- 
nient strengths of which have been fixed after repeated trials with 
solutions containing varying amounts : — 
1. Solution of Permanganate of Potassium — one gram in the litre. — - 
This solution, although recrystallised permanganate is usually almost 
pure, must have its exact strength calculated by titration in one or 
other of the ordinary ways, — preferably, I think, against metallic 
iron or a ferrous salt, although oxalic acid has a powerful advocate 
in Berthelot.J 
2. Solution of Ammonio-ferrous Sulphate . — It is most convenient 
that this solution be of such a strength that 1 c.c. will be peroxidised 
by, and will therefore decolorise 1 c.c. of the permanganate solu- 
tion. To do this we require roughly 12’5 grammes of the iron salt 
in the litre (12*4339 precisely). The mere watery solution soon 
decomposes, but I find the addition of 1 c.c. of pure sulphuric acid 
for each gramme of the salt employed (that is, I add 12 5 c.c. of acid 
for each litre of the solution) keeps the solution well, and also fur- 
nishes acid to assist the acid already added in decomposing the per- 
manganate, and in converting the ferrous into the ferric salt. Each 
day the solution is used it must, however, be titrated against the 
permanganate, a process w T hich occupies only a few minutes. 
3. Diluted Sulphuric Acid .' — That of the British Pharmacopoeia 
does quite well, — it is almost 1 in 12, but I am in the habit of 
usiug a 10 per cent, solution of pure acid. It is very necessary to 
be certain that the solution will not itself decolorise some perman- 
ganate, as impure sulphuric acid so generally does. 
* Journal of the Chemical Society, vol. xxxv. (new series), p. 46-106. 
t Pharmaceutical Journal , vol. x. (3rd series), p. 94. 
£ Dingler’s Polytechnisches Journal, ccxii. p. 354. 
